Coarse rubber particles can be produced by direct emulsion polymerisation, such as is described for example in DE-A 1 247 665 and 1 269 360. However, these direct polymerisation processes have the disadvantage of long polymerisation times, as a rule lasting several days, until the desired particle diameter is achieved at almost complete conversion. In order to keep the polymerisation times as short as possible, high polymerisation temperatures are also often employed. This leads to the formation of Diels-Alder adducts such as vinylcyclohexene that are difficult to remove and that persistently remain in the latex particles.
As an alternative to direct polymerisation coarse rubber particles can be produced by agglomeration of finely particulate latices. The term finely particulate latices is understood to mean those having a mean particle diameter (DVN) of 40 to 250 nm. The more finely particulate the latex, the shorter the polymerisation time. The agglomeration can be initiated by physical and chemical processes. In this connection it is very difficult to avoid the formation of undesirable coagulate, i.e. very large agglomerates (ranging from several μm up to mm size) that separate out from the dispersion and cannot be redispersed. Such coarse fractions also reduce the gloss and adversely affect the mechanical and surface properties of plastics materials, e.g. ABS, produced from the latex.
A chemical method of enlarging rubber latices is described in DE-A 2 606 715, according to which acetic anhydride is added to the rubber latex. The acetic acid released by hydrolysis neutralises the carboxylate emulsifier and destabilises the latex until the rubber particles agglomerate. The process can be used only with weakly acid emulsifiers however, such as for example salts of organic acids. Latices that are stabilised with highly active sulfonate or sulfate emulsifiers cannot be agglomerated by this method. This process also has the disadvantage that the agglomerating latex cannot be stirred during the agglomeration phase on account of its extreme sensitivity to shear forces and has to be stabilised after the agglomeration with acid-stable emulsifiers or alkali. This results in a large amount of effluent. The possibility of performing a continuous agglomeration in particular is excluded. A continuous agglomeration process has the great advantage that the process can be controlled and regulated in the event of disturbances in the agglomeration and/or deviations from the desired mean agglomerate size.
According to the teaching of DE-A 2 645 082 the agglomeration is initiated by oxidised polyethylene oxide. The agglomerated latices have a very broad particle size distribution, which is disadvantageous for example in the production of ABS. Also the latices obtained by this method are only limitedly stable during further processing steps. If unoxidised polyethylene oxide (PEO) is used, ammonium salts must be added (U.S. Pat. No. 3,288,741), resulting in a relatively high effluent burden. In EP-A 330 865 branched polyethylene oxide is used before and/or during the emulsion polymerisation. Here too considerable amounts of alkali salts or ammonium salts must be employed. Also, the use of PEO-containing emulsifiers is described for example in DE-A 2 323 547 (=U.S. Pat. No. 4,014,843) or U.S. Pat. No. 4,680,321. This leads to broad particle size distributions with a considerable proportion of non-agglomerated finely particulate rubber particles, and the formation of coagulate can be avoided only by using auxiliary emulsifiers.
In DE-A 2 427 960 a second latex containing carboxyl or amide groups is used as agglomerating agent. The agglomerated latices have a very broad particle size distribution and contain a considerable proportion of non-agglomerated finely particulate rubber particles. If a latex that is stabilised with a PEO-PS-PEO triblock copolymer is used as agglomerating agent, then according to the teaching of EP-A 249 554 narrow particle size distributions without any coagulate formation are achieved. The production of the latex used as agglomerating agent runs into difficulties however. Both processes involve the expensive production of a second latex, leading to additional costs.